1. Field of the Invention
The present invention relates to active-matrix substrates, electro-optical devices, methods for manufacturing active-matrix substrates, and electronic equipment. More specifically, it relates to an electrostatic-destruction prevention technology suited to manufacturing of an active-matrix substrate of a type in which pixel electrodes are driven by poly-silicon thin-film transistors (thin film transistor is hereinafter referred to as TFT) formed on an insulating substrate.
2. Background of the Related Art
Among various liquid crystal panels, an active-matrix liquid crystal panel is formed, for example, by sequentially and selectively forming a semiconductor layer, an insulating layer, and a conductive layer on a large substrate, such as a glass substrate, to form a plurality of panel areas provided with active elements, passive elements, electrodes, and other components, and by cutting these panel areas from the large substrate. This active-matrix substrate is used for an electro-optical device. Specifically, it is used in an electro-optical device in which a liquid crystal is sandwiched by the active-matrix substrate and an opposing substrate. In the active-matrix substrate, a number of pixels are formed in a matrix and they form a pixel section. In the pixel section, thin-film transistors (TFTs) are formed and a voltage is applied to pixel electrodes through the TFTs.
In such an active matrix substrate, when poly-silicon (Poly-Si) is used as a semiconductor material to form the TFTs, since transistors and other devices constituting peripheral circuits such as a shift register and a driving circuit can be formed in the same process, high integration is enabled.
In such an active matrix substrate, when poly-silicon TFTs are formed as transistors, since the active-matrix substrate can be formed in a low-temperature process, it is advantageous that a glass substrate made from silica glass or non-alkaline glass can be used as an insulating substrate.
Since the glass substrate is likely to be charged, however, when static electricity is discharged from the charged substrate, TFTs and other devices working as active elements may be destroyed (hereinafter called electrostatic destruction) by static electricity.
In an active-matrix substrate, an alignment layer is formed on the glass substrate on which active elements, passive elements, and electrodes are formed, to align liquid-crystal molecules in a prescribed direction. In a rubbing process for the alignment layer, however, the substrate is charged with high-voltage static electricity generated by the rubbing. When static electricity is discharged from the charged substrate, the TFTs and other devices working as active elements may be electrostatically destroyed.
More specifically, an organic high-polymer film made from polyimide resin or the like is formed on the glass substrate on which the active elements and other devices are formed, and the rubbing process is applied to a surface of this resin film to align the liquid-crystal molecules, in which the surface is rubbed with textile fabrics made from fiber, such as rayon and nylon, in a constant direction at a prescribed load. In this process, friction between the resin film and the fiber generates high-voltage static electricity. This static electricity charges the substrate itself, or is discharged over insulation to electrostatically destroy semiconductor devices, such as the TFTs formed on the substrate.
According to knowledge which the inventors of the present application obtained, since poly-silicon TFTs and other devices formed in the low-temperature process at a maximum process temperature of about 400 to 600xc2x0 C. have an extremely low dielectric strength, they are likely to be electrostatically destroyed. In some cases, they may cause a serious problem almost identical to a fatal error, in which the entire driving circuit does not function.
Accordingly, an object of the present invention is to provide an active-matrix substrate, an electro-optical device, and a method for manufacturing an active-matrix substrate, which provide a structure that can effectively prevent TFTs and other devices formed on a substrate from being destroyed by static electricity generated for some reason or by a rubbing process for a liquid-crystal alignment layer.
To achieve the foregoing object, according to the present invention, an active-matrix substrate, on which are formed a pixel section provided with a pixel electrode and a switching element connected to the pixel electrode, a peripheral circuit disposed around the pixel section that controls the switching element, and an external-connection terminal electrically connected to the peripheral circuit, is characterized in that an antistatic conductive layer is formed at least at a part of the area on the substrate excluding the pixel section.
In the present invention, the antistatic conductive layer collects static electricity generated when a rubbing process is applied to a polyimide film formed on the active matrix substrate or the like to change it to a liquid crystal alignment layer, and the charges are dispersed. Therefore, the substrate itself is prevented from being charged. Active elements and other elements formed in the peripheral circuit and other circuits are prevented from being electrostatically destroyed during discharging. Therefore, TFTs formed in a low-temperature process, which are not immune to static electricity, can be used as active elements. In addition, since the antistatic conductive layer serves as a large-capacitance bypass capacitor (xe2x80x9cpass capxe2x80x9d) when an electro-optical device is operated, it contributes to providing lower noise and lower EMI. Therefore, higher image quality and higher resolution are implemented in the electro-optical device.
In the present invention, it is preferred that the antistatic conductive layer be formed in the area on the substrate excluding the pixel section, only at the upper layer sides of a no-wiring section, where wiring is not formed, of an area where wiring is formed to which a DC voltage is applied, and of an area where wiring is formed to which a DC voltage is applied when an image is displayed. With this configuration, even when an antistatic conductive layer is formed, the capacitive load of the driving circuit does not increase. Therefore, since a signal transmitted through the wiring is not delayed, transistors are prevented from being electrostatically destroyed while enabling a high-speed operation.
In the present invention, it is preferred that the antistatic conductive layer be formed such that it is exposed on a surface of the substrate. With this configuration, the antistatic conductive layer positively collects static electricity generated in the rubbing process to disperse it. Therefore, the substrate itself is prevented from being charged, and active elements and other elements are prevented from being electrostatically destroyed during discharging.
In the present invention, it is preferred that the antistatic conductive layer be formed at least at the outer peripheral edge of the substrate. In other words, it is preferred that, after the pixel section, the peripheral circuit, the terminal section, and the antistatic conductive layer are formed in each of a plurality of panel areas, each of which is cut from a large substrate as the active-matrix substrate, the antistatic conductive layer be formed so as to cross over the boundary of adjacent panel areas. With this configuration, since a potential difference between panel areas is eliminated and the same potential plane can be extended, problems caused by static electricity are more positively prevented from occurring.
In the present invention, it is preferred that external-connection terminals be electrically connected through an electrostatic protection circuit in which two sets of diode chains are disposed in reverse directions to each other. It is also preferred that each external-connection terminal and the antistatic conductive layer be connected with an electrostatic protection circuit in which two sets of diode chains are disposed in reverse directions to each other. With this configuration, when the potential of static electricity accumulated on the external-connection terminals exceeds a prescribed value, the static electricity is released through the electrostatic protection circuits to the antistatic conductive layer. Therefore, discharging is prevented from occurring between external-connection terminals and between an external-connection terminal and the antistatic conductive layer.
In the present invention, the switching device and the peripheral circuit may be formed of thin-film transistors. In this case, it is preferred that the channel lengths of diode-connection thin-film transistors constituting the diode chains in the electrostatic protection circuit be longer than the channel lengths of the thin-film transistor connected to the pixel and the thin-film transistors formed in the peripheral circuit. With this configuration, since deterioration of the diode chains used for the electrostatic protection circuit is suppressed, the life of the electro-optical device is extended.
In the present invention, it is preferred that the active-matrix substrate be configured such that the thin-film transistors are connected to a scanning line and a data line; the peripheral circuit includes a data-line driving circuit that outputs to the data line at least an image signal to be applied to the pixel electrode through the thin-film transistor and a scanning-line driving circuit that outputs a scanning signal to control selection/non-selection states of the thin-film transistors to the scanning line; and the plurality of diode-connection external-connection terminals include an external-connection terminal electrically connected to the data-line driving circuit and an external-connection terminal electrically connected to the scanning-line driving circuit. With this configuration, since a potential difference is prevented from being generated between the data-line driving circuit and the scanning-line driving circuit, electrostatic destruction is positively prevented from occurring in an imbalanced manner in either side of the data-line driving circuit or the scanning-line driving circuit.
In the present invention, it is preferred that the antistatic conductive layer is made from the same material as the pixel electrode or the external-connection terminals. It is preferred, for example, that the antistatic conductive layer is made from Al (aluminum), Ti (titanium), Ta (tantalum), Cr (chromium), or an alloy thereof. The antistatic conductive layer may be formed of a transparent conductive film made from an indium tin oxide film (ITO film). With this configuration, since the antistatic conductive layer can be formed at the same time as the pixel electrode or the external-connection terminals, the manufacturing process is simplified. When the antistatic conductive layer is formed of an indium tin oxide film serving as a transparent conductive film, since ITO has a higher resistivity coefficient than other metal materials, instantaneous discharging is received by a circuit network having a longer time constant compared with a case in which the antistatic conductive layer is made from a metal material having almost the same film thickness. Therefore, an instantaneous voltage applied to the wiring during discharging is reduced.
The present invention is effective when the active areas of the thin-film transistors are formed of a poly-silicon film.
An active-matrix substrate according to the present invention is used to form an electro-optical device together with an opposing substrate which opposes the active-matrix substrate with a prescribed gap, and an electro-optical material such as liquid crystal sealed in the gap between the opposing substrate and the active-matrix substrate.
In a method for manufacturing an active-matrix substrate according to the present invention, for example, after the pixel section, the peripheral circuit, the terminal section, and the antistatic conductive layer are formed in each of a plurality of panel areas, each of which is to be cut from a large substrate as the active-matrix substrate, the plurality of panel areas are cut from the large substrate to make the active-matrix substrates.
In this case, it is preferred that the antistatic conductive layer is formed so as to cross over the boundary of adjacent panel areas.
An electronic equipment according to the present invention is characterized in that an electro-optical device is mounted as a display device. An electronic equipment is also characterized in that an electro-optical device is mounted as a light valve.